rayCastTo() returns a reference to an KX_GAmeObject not a string. A string will never equal a KX_GameObject. But the name of the KX_GameObject does:
Hit = scene.objects["CameraBoundary"]
Flare = Sun.rayCastTo(Hit,350,"Player")
Sun["Flare"] = (Flare is not None and Flare.name== "Default")
If you give your “Default” a unique property “Default” you do not even need to check the name. BTW. This looks like a perfect situation for a ray sensor (checking for an object with a specific property).